Reducing data hazards in pipelined processors to provide high processor utilization

ABSTRACT

A pipelined computer processor is presented that reduces data hazards such that high processor utilization is attained. The processor restructures a set of instructions to operate concurrently on multiple pieces of data in multiple passes. One subset of instructions operates on one piece of data while different subsets of instructions operate concurrently on different pieces of data. A validity pipeline tracks the priming and draining of the pipeline processor to ensure that only valid data is written to registers or memory. Pass-dependent addressing is provided to correctly address registers and memory for different pieces of data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/101,902, filed Dec. 10, 2013, which is a divisional of U.S. patentapplication Ser. No. 13/205,552, filed Aug. 8, 2011, issued as U.S. Pat.No. 8,612,728 on Dec. 17, 2013, which is a divisional of U.S. patentapplication Ser. No. 12/782,474, filed May 18, 2010, issued as U.S. Pat.No. 8,006,072 on Aug. 23, 2011, which is a divisional of U.S. patentapplication Ser. No. 11/711,288, filed Feb. 26, 2007, issued as U.S.Pat. No. 7,734,899 on Jun. 8, 2010, which is a continuation of U.S.patent application Ser. No. 10/125,331, filed Apr. 18, 2002, issued asU.S. Pat. No. 7,200,738 on Apr. 3, 2007. These applications and patentsare incorporated herein by reference, in their entirety, for anypurpose.

BACKGROUND OF THE INVENTION

This invention relates to pipelined computer processors. Moreparticularly, this invention relates to pipelined computer processorsthat reduce data hazards to provide high processor utilization.

A processor, also known as a central processing unit, processes a set ofinstructions from a stored program. The processing of an instruction istypically divided into multiple stages, where each stage generallyrequires one clock cycle to complete and typically requires differenthardware within the processor.

For example, the processing of an instruction can be divided into thefollowing stages: fetch, decode, execute, and write-back. At the fetchstage, the processor retrieves an instruction from memory. Theinstruction is typically encoded as a string of bits that representinput information (e.g., operands), an operation code (“opcode”), andoutput information (e.g., a destination address). An opcode representsan arithmetic or logic function associated with the operands. Once theinstruction is retrieved from memory, a program counter is eitherincremented for linear program execution or updated to show a branchdestination. The program counter contains a pointer to an address inmemory from which a next instruction is fetched. At the decode stage,the processor decodes the instruction into an opcode, operands, and adestination. The opcode can include one of the following: add, subtract,multiply, divide, shift, load, store, loop, branch, etc. The operands,depending on the opcode, can be constants, values stored at one or morememory addresses, or the contents of one or more registers. Thedestination can be a register or a memory address where a resultproduced from execution of the opcode is stored. At the execute stage,the processor executes the decoded opcode using the operands. Forinstructions such as add and subtract, the execute stage typicallyrequires one clock cycle. For more complicated instructions, such asmultiply and divide, the execute stage typically requires more than oneclock cycle. At the write-back stage, the processor stores the resultfrom the execute stage at the specified destination.

Pipelining is a known technique that improves processor performance byoverlapping the execution of instructions such that differentinstructions are in each stage of the pipeline during a same clockcycle. For example, while a first instruction is in the write-backstage, a second instruction can be in the execute stage, a thirdinstruction can be in the decode stage, and a fourth instruction can bein the fetch stage. In an ideal situation, one instruction completesprocessing each clock cycle, and processor utilization is 100%.Processor utilization can be determined by dividing the number ofprogram instructions that complete processing by the number of clockcycles in which those instructions complete processing.

Although pipelining can increase throughput (the number of instructionsexecuted per unit time), it increases instruction latency (the time tocompletely process an instruction). Increases in throughput arerestricted by data hazards. A data hazard is a dependence of oneinstruction on another instruction. An example is a load-use hazard,which occurs when the result of one instruction is needed as input for asubsequent instruction. Instructions (1) and (2) below illustrate aload-use hazard. R0, R1, R2, R3, and R4 represent register contents.R0←R1+R2  (1)R3←R0+R4  (2)

In the four-stage pipeline described above, the result of instruction(1) is stored in register R0 and is available at the end of thewrite-back stage. Data dependent instruction (2) needs the contents ofregister R0 at the beginning of the decode stage. If instruction (2) isimmediately subsequent to instruction (1) or is separated frominstruction (1) by only one instruction, instruction (2) will retrievean old value from register R0.

Software techniques that do not require hardware control for reducingsuch data hazards are known. One technique eliminates data hazards byexploiting instruction-level parallelism to reorder instructions. Toeliminate a data hazard, an instruction and its associateddata-dependent instruction are separated by sufficient independentinstructions such that a result from the first instruction is availableto the data-dependent instruction by the start of the data-dependentinstruction's decode stage. However, there is a limit to the amount ofinstruction-level parallelism possible in a program and, therefore, alimit to the extent that data hazards can be eliminated by instructionreordering.

Data hazards that cannot be eliminated by instruction reordering can beeliminated by introducing one or more null (i.e., no-operation or nop)instructions immediately before the data-dependent instruction. Each nopinstruction, which advances in the pipeline, simply delays theprocessing of the rest of the program by a clock cycle. The addition ofnop instructions increases program size and total program executiontime, which decreases utilization (since nop instructions do not processany data). For example, when each instruction is data-dependent on animmediately preceding instruction (such that the instructions cannot bereordered), two nop instructions should be inserted between each programinstruction (e.g., A..B..C, where each letter represents a programinstruction and each “.” represents a nop instruction). The utilizationbecomes less than 100% for a processor running in steady state (e.g.,for instructions A, B, and C, the utilization is 3/7 or 43%; for ninesimilar instructions, the utilization is 9/25 or 36%), which does nottake into account priming or draining. Priming is the initial entry ofinstructions into the pipeline and draining is the clearing ofinstructions from the pipeline.

In addition to software techniques, hardware techniques, such as “dataforwarding,” are known. Without data forwarding, the result of aninstruction, which is known at the end of the execute stage, is notavailable as input to another instruction until the end of thewrite-back stage. Data forwarding forwards that result one cycle earlierso that the result is available as input to another instruction at theend of the execute stage. With data forwarding, an instruction onlyneeds to be separated from a data-dependent instruction by oneindependent instruction or one nop instruction. For example, inhardware, a state register R0 can hold a register value X. Without dataforwarding, a new value Y can be written into R0 during a cycle n (e.g.,a write-back stage) such that Y is available at a next cycle (n+1).Because the new value Y may be needed by an instruction in cycle n,control logic associated with data forwarding enables a multiplexer tooutput the new result Y, making Y available for another instruction onecycle earlier (cycle n). While data forwarding advantageously providesdata one cycle earlier (which improves processor utilization), dataforwarding hardware requires additional circuit area which increasescost. Data forwarding also increases hardware complexity, whichincreases design and verification time.

Furthermore, many hazards cannot be resolved by data forwarding (e.g.,cases in which a new value cannot be forwarded). In these instances,stalling the pipeline is an alternative hardware method. Stalling allowsinstructions ahead of a data-dependent instruction to proceed while theprocessing of that data-dependent instruction is stalled. Once thehazard is resolved, the stalled section of the pipeline is restarted.Stalling the pipeline is analogous to the software technique ofintroducing nop instructions, except that the hardware stallingtechnique is automatic and avoids increasing program size. However,stalling also reduces performance and thus utilization.

In view of the foregoing, it would be desirable to provide a pipelinedprocessor that reduces data hazards such that high processor utilizationis attained.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a diagram illustrating generally the processing of aninstruction in multiple stages;

FIG. 2 is a diagram illustrating a known pipelining of multipleinstructions without data hazards;

FIG. 3 is a diagram illustrating a known pipelining of multipleinstructions with data hazards;

FIG. 4 is a diagram illustrating the pipelining of subsets ofinstructions in multiple passes for a single piece of data in accordancewith the invention;

FIG. 5 is a diagram illustrating the pipelining of subsets ofinstructions in multiple passes for three pieces of data in accordancewith the invention;

FIG. 6 is a diagram illustrating alternatively the pipelining of FIG. 5;

FIG. 7 is a diagram illustrating the pipelining of subsets ofinstructions in multiple passes for many pieces of data in accordancewith the invention;

FIG. 8 is a diagram illustrating alternatively the pipelining of FIG. 7:

FIG. 9 is a table illustrating the priming and draining of a validitypipeline for multiple pipeline passes in accordance with the invention:

FIG. 10 is a table illustrating a preferred arrangement ofpass-dependent register file addressing in accordance with theinvention;

FIG. 11 is a table illustrating a more preferred arrangement ofpass-dependent register file addressing in accordance with theinvention;

FIG. 12 is a table illustrating register mapping in accordance with theinvention;

FIG. 13 is a table illustrating frame pointers in a validity pipeline inaccordance with the invention;

FIG. 14 is a table illustrating address mapping in accordance with theinvention; and

FIG. 15 is a block diagram of a pipelined processor in accordance withthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the processing of an instruction in a fetch stage (F)102, a decode stage (D) 104, an execute stage (E) 106, and a write-backstage (WB) 108. Although process 100 shows only four stages ofinstruction processing (for clarity), an instruction can be processed inother numbers and types of stages.

In a pipeline that processes multiple instructions with no data hazards(which can thus run at maximum utilization), an instruction enters thepipeline at each clock cycle and propagates through each stage with eachsubsequent clock cycle. Upon a first instruction completing thewrite-back stage, an instruction ideally completes processing everyclock cycle thereafter. FIG. 2 illustrates the pipelining of multipleinstructions (which have a 1-cycle execute stage) with no data hazards.When the processor is in steady state (i.e., when the first instructionis being processed in a last stage of the pipeline; e.g., wheninstruction A is in cycle 4), processor utilization is 100%. For morecomplicated instructions, such as multiply and divide, which requiremore than one clock cycle for the execute stage, an instruction may notbe able to complete processing every cycle and, thus, maximumutilization may be less than 100%.

In a pipeline that processes multiple instructions with data hazards,the hazards can be avoided with software that inserts nop instructionsbetween dependent instructions. However, this increases programexecution time. FIG. 3 illustrates the pipelining of multipleinstructions where each instruction is data-dependent on an immediatelypreceding instruction.

The invention provides a pipelined processor that reduces data hazardsto improve processor utilization by executing a set of instructions inmultiple passes concurrently, with subsets of the instructions operatingon different pieces of data. The type of program preferably processed bythe invention is one that is repeatedly executed (e.g., on differentsets of input data) until a predefined condition occurs (e.g., a counterreaches a predefined number). A program's set of instructions isprocessed in multiple passes. In each pass, one or more subsets of thoseinstructions are processed. For example, a program of nine instructionsmay be processed in three passes, with subsets of three instructionsbeing processed in each pass. This technique, called “softwarepipelining,” is advantageous for processing many different sets of inputdata with a fixed algorithm. The more sets of input data, the morelikely the processor is to run at maximum utilization, which may be lessthan 100% depending on the number of cycles required to process eachinstruction.

In accordance with the invention, a program is first restructured (e.g.,by a specialized compiler) before being run on a software pipelinedprocessor. The program's instructions are first preferably expressed asa single instruction sequence without data-dependent branches. Branchescan be eliminated by providing an instruction set that allows predicatedexecution. Predicated execution is the conditional execution ofinstructions based on a boolean (e.g., true or false) value known as apredicate. These instructions may be executed depending on whether ornot a branch would be taken. When the predicate is true, theinstructions execute normally and the results, if any, are written tomemory or to registers. However, when the predicate is false, theinstructions are skipped (i.e., not executed). Second, data hazards areremoved by reordering instructions and inserting nop instructions. Atthis point, the processor will operate at less than maximum utilizationbecause of the nop instructions. Third, the sequence of instructions isdivided into a number of subsets and interleaved so that all of the nopinstructions are replaced by instructions from different subsets. Adetailed example of this process is now described.

The instruction sequence A, B, C, D, E, F. G, H, and I (nineinstructions) processes a single piece of input data, where eachinstruction requires two subsequent nop instructions to eliminate datahazards. (Note that the invention is not limited to regular sequences ofthis type, but is advantageously applicable to instruction sequenceswith arbitrary data hazards on arbitrary instructions within thesequence.) The execution sequence for this example is expressed as asingle instruction sequence without data hazards in accordance with theinvention as follows:A..B..C..D..E..F..G..H..I  (3)where each period represents a nop instruction. This sequence is thendivided into three subsets of instructions in accordance with theinvention as follows:Subset I: A..B..C..  (4)Subset II: .D..E..F.  (5)Subset III: ..G..H..I  (6)The subsets are arranged so that each subset contains a linearsequential fragment of the original sequence, and each of the subsetsare of equal length. If necessary, the lengths of the subsets can bemade equal by the addition of nop instructions. As shown, two additionalnop instructions are introduced (one between instructions C and D, andone between instructions F and G). The optimum number of subsetsrequired varies from program to program. For a given program, increasingthe number of subsets allows processor utilization to increase until itreaches a maximum (which may be less than 100%). This maximum is reachedwhen the number of nop instructions in the program reaches a minimum.Increasing the number of subsets beyond this point will require theaddition of nop instructions and will therefore decrease the processorutilization.

FIG. 4 illustrates the software pipelining of these three subsets ofinstructions operating on a single piece of data in three passes whileavoiding data hazards. Note that only the first stage (e.g., the fetchstage) of the hardware pipeline for each instruction is represented inFIG. 4 for clarity. Instruction A enters the fetch stage at a firstcycle of pass 1. After instruction C enters the fetch stage, twoadditional nop instructions are processed in pass 1. Instruction Denters the fetch stage at a second cycle of pass 2. After instruction Fenters the fetch stage, an additional nop instruction is processed inpass 2. Instruction G enters the fetch stage at a third cycle of pass 3.Instruction I is the final instruction that enters the pipeline in pass3.

The processor preferably runs a set of instructions on multiple piecesof data concurrently, with each subset of instructions operating ondifferent pieces of data during the same pass, in accordance with theinvention. Using the example above with three pieces of data, in pass 1,subset I operates on a first piece of data. In pass 2, subset 11operates on the first piece of data while subset I begins operating on asecond piece of data. Subsets I and II are preferably interleaved in thehardware pipeline. This can be represented using the notationA(2)D(1)G(-)B(2)E(1)H(-)C(2)F(1)I(-) where the number in parentheses (n)represents the ordinal number of the piece of data being processed and(-) represents a subset that is not yet executing valid data and istherefore executing nop instructions. In passes 1 and 2, the softwarepipeline is primed as more subsets of instructions are processing validdata, reducing the number of nop instructions processed in the hardwarepipeline. In pass 3, subset III operates on the first piece of data,while subset II operates on the second piece of data and subset Ioperates on a third piece of data. The instructions are again preferablyinterleaved in the hardware pipeline (e.g.,A(3)D(2)G(1)B(3)E(2)H(1)C(3)F(2)I(1)). At this point, the softwarepipeline is fully primed and the processor is advantageously running atmaximum utilization. One instruction completes processing each cycle.This continues until each piece of data has been processed through eachsubset of instructions. As subsets of instructions finish processing thefinal piece of data, the software pipeline drains as nop instructionsenter the hardware pipeline.

FIG. 5 illustrates the above pipelining of three subsets of instructionsoperating on three pieces of data. In pass 1, the processor beginsprocessing a first piece of data (designated as subscript “1” next to acorresponding instruction). In pass 2, the processor begins processing asecond piece of data (designated as subscript “2”). In pass 3, theprocessor begins processing a third piece of data (designated assubscript “3”). By pass 4, the processor has finished processing thefirst piece of data through all subsets of instructions. By pass 5, theprocessor has finished processing the second piece of data through allsubsets of instructions and is processing the third (final) piece ofdata through the last subset of instructions (Subset III). In passes 1and 2, the nop instructions represent the priming of the softwarepipeline. In pass 3, the processor is running at maximum utilization,and in passes 4 and 5, the nop instructions represent the draining ofthe software pipeline. In both cases, the nop instructions are actuallyvalid instructions that have been inhibited because there is no validdata to act upon.

FIG. 6 is another illustration of pipelining 500. The instructions areprocessed from left (instruction A) to right (instruction I) and fromtop (pass 1) to bottom (pass 5). An instruction not operating on anydata (i.e., an instruction that is behaving as a nop instruction) isdesignated by a “(-)” next to that instruction and preferably occursonly during priming and draining of the software pipeline.

FIG. 7 illustrates the pipelining of three subsets of instructionsoperating on several pieces of data in accordance with the invention.The more pieces of data that are processed in the pipeline, the morelikely the overall performance of the processor is to approach maximumutilization.

The utilization attained when processing a sequence of instructions isdependent upon the following: the total number of instructions in aprogram, the number of subsets into which the program is divided, andthe number of pieces of data processed. For the 3-subset example above,the utilization is given by equation (7):

$\begin{matrix}{{100*\frac{\left( {\left( {\left( {\left( {N + 2} \right)*I} \right) - I} \right) - I} \right)}{\left( {\left( {N + 2} \right)*I} \right)}} = {100*\frac{N}{N + 2}}} & (7)\end{matrix}$where “I” is the number of instructions in the program and “N” is thenumber of pieces of data. In reduced form, utilization depends on onlyN. Because the value of the numerator (N) will always be less than thevalue of the denominator (N+2), utilization will be less than 100%. Thisoccurs because of nop instructions during priming and draining of thesoftware pipeline. However, as N increases, utilization approaches 100%(i.e., for very large N, the “+2” becomes negligible and (N+2)approximately equals N).

FIG. 8 illustrates pipelining N pieces of data in accordance with theinvention. To properly control the software pipeline, the number ofsubsets (NumberOfSubsets) is preferably programmed into a registerbefore a program starts processing. To process N pieces of data,(N+NumberOfSubsets−1) passes are required, with the software pipelinebeing primed during a first (NumberOfSubsets−1) passes and being drainedduring a last (NumberOfSubsets−1) passes. For example, for the 3-subsetexample above, the number of passes needed to process 6 (N=6) pieces ofdata is 8 (i.e., 6+3−1). The software pipeline is primed during thefirst 2 (i.e., 3−1) passes and drained during the last 2 passes.

A “LOOP” mechanism in accordance with the invention causes a firstprogram instruction of a subset to begin operating on a next piece ofdata at the start of a next pass. For example, as shown in FIG. 8, theloop mechanism causes instruction A to begin operating on a second pieceof data at the start of pass 2. This mechanism can be implementedexplicitly (e.g., by having a LOOP instruction encoded as one of theinstructions in the program) or implicitly (e.g., by detecting when aprogram counter has reached an “end-of-program” value). The effect isthe same in both cases: the program counter is typically reset to theaddress of the first instruction of the first subset, causing the firstsubset to begin executing on a new piece of data.

The LOOP instruction has several other functions. When the loopinstruction is executed, a current pass counter can be incremented,modulo NumberOfSubsets. The modulo function divides the counter value byNumberOfSubsets and stores the integer remainder in the current passcounter. The current pass counter keeps track of a current piece of datathrough a given loop. In addition, a validity pipeline can be advanced.

The validity pipeline is a hardware pipeline that uses preferably one ormore bits to keep track of valid data in the software pipeline. Inparticular, it is used to track the priming and draining of the softwarepipeline. The validity pipeline has a number of stages equal toNumberOfSubsets and a validity bit associated with each stage. Eachpiece of data can be associated with a validity bit (V) that propagatesalong the validity pipeline. When a program begins, the validity bit foreach stage is initially cleared. When a first subset of instructions (ina first pass) begins processing a first piece of data, a validity bitassociated with the first piece of data is set (e.g., to “I”) and entersa first stage of the validity pipeline. When a second subset ofinstructions (in a second pass) begins processing the first piece ofdata, the validity bit propagates to a second stage of the validitypipeline. Concurrently, when the first subset of instructions beginsprocessing a second piece of data, a validity bit associated with thesecond piece of data is set (e.g., to “1”) and enters the first stage ofthe validity pipeline.

A data write that changes the state of a system (e.g., by writing to adestination register or to memory) is preferably only allowed if it iscaused by an instruction associated with a valid bit (e.g., a bit of“1”). All other writes should be inhibited. This mechanism can causenormal instructions to behave like nop instructions at certain times,particularly during priming and draining of the software pipeline. Whilereads that have no effect on the state of the system do not need to beinhibited, system performance may be improved by eliminating unnecessaryreads associated with invalid passes. When there is no new input data(i.e., the last piece of data has already entered the softwarepipeline), a cleared validity bit enters the first stage of the validitypipeline. The program stops processing when the validity bits in eachstage are cleared.

FIG. 9 illustrates the priming and draining of a 3-stage(NumberOfSubsets=3) validity pipeline for the processing of three piecesof data (N=3). Five passes (i.e., N+NumberOfSubsets−1=5) are required tocompletely process the data (see, e.g., FIG. 5). Stage 1 is associatedwith a first pass for a given piece of data stage 2 is associated with asecond pass, and stage 3 is associated with a third pass. At the startof a program, validity bits in each stage are cleared. A first subset ofinstructions begins processing a first piece of data in pass 1, a secondsubset of instructions in pass 2, and a third subset of instructions inpass 3. This causes a validity bit to be set (e.g., to “1”) in stage 1during pass 1, which propagates to stage 2 in pass 2 and then to stage 3in pass 3. By the start of pass 4, the validity bit for stage 1 is reset(e.g., to “0”), because a final piece of data had already entered thesoftware pipeline in pass 3. This cleared validity bit propagates downthe pipeline with each subsequent pass. When all the validity bits arecleared, the processor preferably prevents the start of another pass tosave power. Program execution can then be stopped under hardwarecontrol.

Because a subset of instructions operating on a particular piece of datacan be interleaved with other subsets of instructions operating ondifferent pieces of data, new data-dependent addressing modes arepreferably implemented in accordance with the invention for someprocessor and system resources (e.g., memory and general-purposeregisters). Global resources (e.g., constants), however, can still beaccessed in a data-independent way.

There is preferably a separate set of registers allocated for each pieceof data processed by program instructions and an addressing mechanism toaccess those registers correctly. For example, for a three-subsetprogram, there are preferably three sets of registers: one setassociated with each of the first three pieces of data in the softwarepipeline. For a fourth piece of data to be processed in a fourth pass,the set of registers allocated for the first piece of data can be reusedfor the fourth piece of data (because the first piece of data hascompletely processed). This is known as pass-dependent register fileaddressing.

There are two ways of implementing pass-dependent register fileaddressing in accordance with the invention. One approach is to allocatea group of physical registers for each piece of data. Each group ofphysical registers is associated with a parallel set of temporaryregisters, which can be addressed by the program. The number oftemporary registers (NumberOfTemporaries) is typically programmed at thestart of program execution. This approach does not require thatNumberOfSubsets be known. A more preferred second approach is toallocate a group of physical registers equal to NumberOfSubsets, withthe same temporary register number assigned to each physical register ineach group, but for a different pass of the program. In both approaches,the number of physical registers required is equal to(NumberOfSubsets*NumberOfTemporaries), and should not exceed the numberof registers available.

FIG. 10 illustrates pass-dependent register mapping 1000 in which eachgroup of physical registers (e.g., 1002, 1004, 1006) is associated witha different piece of data. “Pass Used” indicates the pass in which afirst subset of instructions for a given piece of data is processed(e.g., a fourth piece of data in a 3-subset program uses the group ofphysical registers associated with pass 1). “Register Name” indicatesthe temporary register name that can be addressed by the program.

FIG. 11 illustrates a more preferred pass-dependent register mapping1100 in which each group of physical registers (e.g., 1102, 1104)contains a number of registers equal to NumberOfSubsets. Each registerin groups 1102 and 1104 is assigned the same temporary register name butfor different pieces of data. As before, “Register Name” indicates thetemporary register name that can be addressed by the program. For a3-subset program, register R0 is mapped to physical register numbers 0,1, and 2 for passes 1, 2, and 3, respectively, at 1102.

Instructions operating on a particular piece of data during differentpasses (subsets) may need to access the same physical register usingpass-dependent register file addressing. Using the more preferredregister mapping (FIG. 11), the physical register number can becalculated using equation (8) below. Calculation of the physicalregister number preferably occurs in the instruction decode stage of thehardware pipeline.Physical Register=(Register*NumberOfSubsets)+(CurrentPass−PassUsed) %NumberOfSubsets  (8)where “Register” is the temporary register number (e.g., 0 for R0, 1 forR1); “PassUsed” is the pass number for a particular subset ofinstructions for a given piece of data (e.g., a first subset ofinstructions has PassUsed=1, a second subset of instructions hasPassUsed=2, a third subset of instructions has PassUsed=3); and symbol“%” represents the modulo operator. Register and PassUsed are typicallyinvariant for a particular instruction and are preferably encoded withinthe operands of the instruction. The value of “NumberOfSubsets” is fixedfor a given program. “CurrentPass” is the pass at which a piece of databegins processing (e.g., for a 3-subset program, a first piece of datahas CurrentPass=1, a second piece of data has CurrentPass=2, a thirdpiece of data has CurrentPass=3, a fourth piece of data hasCurrentPass=1). As the processor processes successive passes, itmaintains the value of CurrentPass by incrementing a counter. When thecounter reaches NumberOfSubsets, the counter resets to 1. As a result,CurrentPass is a number between 1 and NumberOfSubsets (i.e.,1<CurrentPass<NumberOfSubsets).

FIG. 12 illustrates physical register mapping 1200 for temporaryregister R1 using the 3-subset program of FIG. 7 in accordance with themore preferred mapping arrangement of FIG. 11. For example, consider theprocessing of a first piece of data by instructions A, D, and G, andsuppose that all three instructions require access to register R1. FIG.7 shows that this processing occurs in passes 1, 2, and 3, respectively.The operands for instructions A, D, and G all encode a register value of1, and encode a PassUsed value of 1, 2, and 3, respectively. Wheninstruction A processes the first piece of data in pass 1, equation 1202shows that physical register 3 is addressed. Equation 1202 shows thedifferent values used to calculate the physical register number usingequation (8). When instruction A processes a second piece of data inpass 2, equation 1202 shows that physical register 4 is addressed. Wheninstruction D processes the first piece of data in pass 2, equation 1202shows that physical register 3 is addressed. Similarly, when instructionG processes the first piece of data in pass 3, equation 1202 shows thatphysical register 3 is addressed. Thus different passes in which thesame piece of data is processed can share the same temporary registers.In equation 1202, CurrentPass is the only invariant term for a giveninstruction. As CurrentPass changes for different passes, a giveninstruction accesses a same group of physical registers. Because eachpiece of data accesses a different physical register in the same group,different pieces of data can be independently processed.

In addition to pass-dependent register addressing, it may also benecessary for the program flow associated with a particular piece ofdata to perform memory reads and writes. A form of pass-dependent memoryaddressing is therefore provided in accordance with the invention.Because multiple subsets of instructions preferably operate concurrentlyon different pieces of data, memory locations corresponding to eachpiece of data are preferably known before a pass starts. For example, ifeach piece of input data causes the program to perform three writes, itmay be necessary to be able to determine a base address for the threewrites as a function of an ordinal number of a piece of data. The memoryaddress can be calculated by summing at least two values: one a functionof the ordinal number of a piece of data and the other a function of theparticular write which is preferably encoded within the programinstructions.

For example, if a piece of code generates three outputs for each pieceof input data, these outputs can be stored sequentially in memory atoffsets 0, 1, and 2 from a base address. Writing to offsets 0, 1, and 2can occur in any order and the value of each offset (0, 1, 2) can beencoded within the stored instruction.

The address for storing the outputs generated by a piece of code can becalculated by adding a base address (which differs for each piece ofinput data) to an offset (e.g., the offset can be “0” for a firstoutput. “1” for a second output, and “2” for a third output), which ispreferably encoded within the operands of an instruction. Before aprogram starts, the base address is preferably set and the number ofoutputs for each input is preferably specified. Each piece of input datacan have an associated base address for outputs as shown in (9) below.Data 0→base address(x)Data 1→base address(x+3)Data 2→base address(x+6)  (9)

The base address for each subsequent piece of data is preferablyincremented by three to allow storage space for the three outputs fromeach piece of input data. Alternatively, each piece of data may beassigned a unique base address independent of the base addresses forother pieces of data. The number of separate copies of the base addressthat are maintained equals NumberOfSubsets. These stored values arecalled “frame pointers” and are preferably stored in a field within thevalidity pipeline.

When a valid pass starts, the current value of the base address can beplaced into the frame pointer field of stage 1 of the validity pipeline,with a corresponding validity bit set (e.g., to “1”). For invalid passes(V=0), particularly during the priming and draining of the softwarepipeline, the value of the frame pointer field is irrelevant, since acleared validity bit (V=0) will inhibit any write instruction, forcingit to act as a nop. The value of the base address is only incrementedafter it has been assigned to a frame pointer field associated with avalid pass (V=1). Meanwhile, the previous base address is propagateddown the frame pointer fields of the validity pipeline to a stage 2associated with a second subset of instructions for the same piece ofdata. The size of the output data is preferably not determined duringprogram execution time, but calculated during “compile” time when theprogram is restructured and the NumberOfSubsets is determined.

FIG. 13 illustrates frame pointers in a validity pipeline (as shown inFIG. 9) in accordance with the invention. Frame pointers (x), (x+3), and(x+6) are associated with valid bits in the validity pipeline. Framepointers “D/C” (don't care) and (x+9) are associated with invalid bitspertaining to the priming and draining of the software pipeline. No datais stored at these addresses during these passes.

When an instruction performs a load or a store, it preferably specifiesthe pass in which the load or store is to be done and can also be usedto select an associated frame pointer in the validity pipeline. Forexample, if instructions A. D, and G (in passes 1, 2, and 3,respectively) are to access memory at offsets 2, 0, and 1, respectively,the physical address can be calculated by selecting the appropriateframe pointer from the validity pipeline and adding the frame pointervalue to the respective offsets.

FIG. 14 illustrates how the frame pointer values are extracted from thevalidity pipeline associated with FIG. 13. The offset indicates thelocation in memory from the base address, and the notation (Frame(n)=m)shows the current value m of the frame pointer field in the nth stage ofthe validity pipeline. The shaded regions indicate loads or stores thatare inhibited because of invalid bits (e.g., V=0) for a given pass. FIG.14 shows how instructions A, D, and G are encoded to perform stores intheir respective passes 1, 2, and 3. By accessing the 1st, 2nd, and 3rdentries in the validity pipeline, the instructions can access the regionof memory associated with the same copy of the base address.

Invalid passes (e.g., when V=0) occur during priming and draining of thesoftware pipeline, and may occur during processing, particularly whenhandling periodic gaps of input data. If input data is available for anew pass, the validity bit is set and the pass proceeds as normal. Ifinput data is not available when the pass starts, the validity bit iscleared for that pass and the pass can still proceed. If input data isavailable for the next pass, there will be a one-pass “bubble” in thesoftware pipeline, represented by the cleared validity bit. If inputdata is not available for a time equal to NumberOfSubsets (indicatingthat all instructions have completely processed the last piece of data),each stage of the validity pipeline will have their validity bitscleared, indicating that the software pipeline has completely drained.

FIG. 15 illustrates a pipelined processor 1500 in accordance with theinvention. During an initial setup, a set of instructions from a programare loaded into a local program memory 1502. Also, initial values areloaded into the following: a program counter 1504, a base addressregister 1506, a Number-Of-Outputs register 1508, and aNumber-Of-Subsets register 1510. In addition, validity bits in avalidity pipeline 1512 are cleared and a current pass counter 1514 isset to zero. Furthermore, validity pipeline 1512 is configured to behaveas though it has a number of stages equal to the value loaded inNumber-Of-Subsets register 1510.

The address of a current instruction in local program memory 1502 isstored in program counter 1504. After the current instruction isfetched, the value in program counter 1504 is updated to reference anext instruction. Control logic 1516 fetches and decodes the currentinstruction from local program memory 1502. The current instruction isprocessed in an instruction pipeline 1518, which preferably contains anumber of (hardware pipeline) stages to process each instruction.Instruction pipeline 1518 can process input data and data reads fromgeneral-purpose registers 1520.

Control logic 1516 controls instruction pipeline 1518 and validitypipeline 1512. Instruction pipeline 1518, validity pipeline 1512, andcontrol logic 1516 are preferably all coupled to a clock 1522, whichsynchronizes their operation. Each time the program code in programmemory 1502 executes a LOOP function, Current-Pass-Counter 1514 isincremented modulo the value in Number-Of-Subsets register 1510,validity pipeline 1512 is advanced, and the current value of baseaddress register 1506 is introduced into the frame pointer field of thefirst entry of validity pipeline 1512. When the LOOP function isexecuted and new input data is available, the value introduced into thevalid field of the validity pipeline is a “1” and the value in baseaddress register 1506 is incremented by the value in Number-Of-Outputsregister 1508. When the LOOP function is executed and no new input datais available, the value introduced into the valid field of the validitypipeline is “0” and the value in base address register 1506 is notmodified. Instruction pipeline 1518 reads/writes data from/to eithergeneral purpose registers 1520 using pass-dependent or pass-independentregister file addressing, or a memory 1524 using pass-dependent orpass-independent memory addressing.

Thus it is seen that data hazards in pipelined processors can be reducedsuch that high processor utilization is attained. One skilled in the artwill appreciate that the invention can be practiced by other than thedescribed embodiments, which are presented for purposes of illustrationand not of limitation, and the invention is limited only by the claimswhich follow.

What is claimed is:
 1. A method, comprising: in a first pass, operatingon a first piece of data in accordance with an instruction of a firstsubset of instructions, wherein a first register of a group of registersof a processor is accessed responsive to a particular register valueencoded in the instruction of the first subset; in a second pass,operating on a second piece of data in accordance with the instructionof the first subset, wherein a second register of the group of registersis accessed responsive to the particular register value encoded in theinstruction of the first subset; and in the second pass, operating onthe first piece of data in accordance with an instruction of a secondsubset of instructions, wherein the first register is accessedresponsive to the particular register value encoded in the instructionof the second subset.
 2. The method of claim 1, wherein the instructionof the first subset is executed responsive to a first cycle of thesecond pass and wherein the instruction of the second subset is executedresponsive to a second cycle of the second pass.
 3. The method of claim1, wherein the first and second subsets are of equal length.
 4. Themethod of claim 1, further comprising: in a third pass: operating on athird piece of data in accordance with the instruction of the firstsubset, wherein a third register of the group of registers is accessedresponsive to the particular register value encoded in the instructionof the first subset; operating on the second piece of data in accordancewith the instruction of the second subset of instructions, wherein thesecond register is accessed responsive to the particular register valueencoded in the instruction of the second subset; and operating on thefirst piece of data in accordance with an instruction of a third subsetof instructions, wherein the first register is accessed responsive tothe particular register value encoded in the instruction of the thirdsubset.
 5. The method of claim 4, wherein the instruction of the firstsubset is executed responsive to a first cycle of the third pass,wherein the instruction of the second subset is executed responsive to asecond cycle of the third pass, and wherein the instruction of the thirdsubset is executed responsive to a third cycle of the third pass.
 6. Themethod of claim 5, wherein operating on a third piece of data inaccordance with the instruction of the first subset includes operatingon the third piece of data in accordance with a first instruction of thefirst subset, wherein the method further includes operating on the thirdpiece of data in accordance with a second instruction of the firstsubset, wherein the third register is accessed responsive to theparticular register value encoded in the second instruction of the firstsubset, and wherein the second instruction of the first subset isexecuted responsive to a fourth cycle of the third pass.
 7. The methodof claim 4, further comprising: in a fourth pass: operating on a fourthpiece of data in accordance with the instruction of the first subset,wherein the first register is accessed responsive to the particularregister value encoded in the instruction of the first subset; operatingon the third piece of data in accordance with the instruction of thesecond subset of instructions, wherein the third register is accessedresponsive to the particular register value encoded in the instructionof the second subset; and operating on the second piece of data inaccordance with the instruction of the third subset of instructions,wherein the second register is accessed responsive to the particularregister value encoded in the instruction of the third subset.
 8. Themethod of claim 1, further comprising, in the first pass, inhibiting theinstruction of the second subset of instructions.
 9. The method of claim1, wherein the first and second subsets are interleaved in a hardwarepipeline of the processor.
 10. The method of claim 1, wherein operatingon a first piece of data in accordance with an instruction of a firstsubset of instructions includes setting a validity bit associated withthe first piece of data.
 11. The method of claim 10, wherein operatingon the first piece of data in accordance with an instruction of a secondsubset of instructions includes propagating the set validity bit from afirst stage of a validity pipeline of the processor to a second stage ofthe validity pipeline; and wherein operating on a second piece of datain accordance with the instruction of the first subset includes settinga validity bit associated with the second piece of data when the firstsubset begins processing the second piece of data.
 12. The method ofclaim 11, further comprising: allowing a data write that changes a stateof the apparatus if caused by an instruction associated with a setvalidity bit; and inhibiting the data write that changes the state ofthe apparatus if caused by an instruction associated with a clearedvalidity bit.
 13. A system for operating a pipelined computer processor,said system comprising: an instruction pipeline that stores a set ofinstructions; and a processor pipeline coupled to the instructionpipeline, the processor pipeline configured to: receive the set ofinstructions from the instruction pipeline; divide the set ofinstructions into a first subset of instructions and a second subset ofinstructions; retrieve an instruction of the first subset ofinstructions and an instruction of the second subset of instructions; ina first pass, process the instruction of the first subset ofinstructions operating on a first piece of data; in a second pass,process the instruction of the first subset operating on a second pieceof data; and in the second pass, process the instruction of the secondsubset operating on the first piece of data.
 14. The system of claim 13,further comprising a group of registers coupled to the processorpipeline: wherein a first register of the group of registers is accessedin the first pass responsive to a particular register value encoded inthe instruction of the first subset; and wherein the first register isaccessed in the second pass responsive to the particular register valueencoded in the instruction of the second subset.
 15. The system of claim13, further comprising control logic operative to assign a differenttemporary register value to each physical register belonging to acorresponding one of the first and second subsets.
 16. The system ofclaim 15, wherein the temporary register value is assigned to a physicalregister belonging to the first subset in the first pass, and assignedto the physical register belonging to the second subset in the secondpass.
 17. A method, comprising: in a first pass, operating on a firstpiece of data in accordance with an instruction of a first subset ofinstructions, wherein a first register of a group of registers isaccessed responsive to a particular register value encoded in theinstruction of the first subset; in a second pass, operating on a secondpiece of data in accordance with the instruction of the first subset,wherein a second register of the group of registers is accessedresponsive to the particular register value encoded in the instructionof the first subset; and assigning a different temporary register valueto each physical register belonging to a corresponding one of the firstand second subsets.
 18. The method of claim 17, wherein the first andsecond subsets of instructions are included in a set of instructions;and wherein each subset includes a number of instructions from the setof instructions.
 19. The method of claim 18, wherein each instruction ofthe set belongs to only one subset.
 20. The method of claim 17, whereinthe instruction of the first subset is executed responsive to a firstcycle of the second pass; wherein the instruction of the second subsetis executed responsive to a second cycle of the second pass; and whereinthe first and second subsets are of equal length.